1. Field of the invention
The present invention relates, in general, to relays. More particularly, the invention relates to a D.C. relay with a power reducing function, which is used in the control circuit of a cooking apparatus, such as a microwave oven.
2. Description of the prior art
Generally, as is illustrated in FIG. 1, a typical D.C. relay includes of an L-shaped base, an iron core mounted on the base, an exciting coil wound around the iron core, a movable contact plate supported at an upper part of the base and over the iron core, a movable contact attached to the movable contact plate and electrically connected with the a fixed terminal of the circuit, a fixed contact facing the movable contact, connected with another fixed terminal of the circuit, and a spring for biasing the movable contact plate to open the contacts of the relay.
In this well-known D.C. relay, when a D.C voltage is applied to the exciting coil, the iron core is magnetized and a force of attraction between the movable contact plate and the iron core is provided. By this force of attraction. the contacts of the relay are closed. Consequently, the fixed terminals of the circuit also are closed. When the D.C. voltage applied to the exciting coil is shut off, the force of attraction between the iron core and the movable contact plate dissipates, and the contacts of the relay are biasing force of the spring. Consequently, the fixed terminals of the circuit are opened.
As is also well known, cooking apparatus, such as, e.g, microwave ovens, have D.C. relays as mentioned above in their control circuits for operating appropriate devices such as fans, heaters, magnetrons and so on.
At the present time, a cooking apparatus is required to have many functions. For example, a microwave oven typically has not only the capability of warming food with microwaves from a magnetron, but also the capability of roasting food with an electric heater.
The greater the number of functions of cooking apparatus is, the greater the number of D.C. relays which must be used in the control circuit thereof. The greater the number of D.C. relays used in the control circuit, the more electric power is consumed in the control circuit. This is because the power consumption of a D.C. relay generally is constant at all times.
Therefore, to supply more power to the control circuit when more functions of cooking apparatus are present, the power supplying transformer Of the control circuit must be larger. This results in a larger and more expensive apparatus.
In order to solve the problem mentioned above, a relay control circuit has been developed which reduces the power consumption of a D.C. relay by decreasing the D.C. power required for holding the relay in a closed state.
The examples of such relay control circuits are disclosed in Japanese Utility Model Publication No. 29152, filed in Feb. 18, 1977 in the name of Masaaki Ishikawa, etc., and in Japanese Utility Model Publication No. 25157, filed in Nov. 24, 1976 in the name of Shigeki Kitamura, etc, respectively.
In Japanese Utility Model Publication No. 29152, a positive pulsating voltage is generated by algebraically adding a half-wave rectified A.C. voltage to a D.C. voltage. And the D.C relay is driven by feeding this positive pulsating voltage at a positive potential with respect to the D.C. voltage, and the D.C relay is maintained in the closed state by the D.C. voltage.
In Japanese Utility Model Publication No. 25157, the D.C relay is closed by an activating D.C. current higher than a holding D.C. current, and is maintained in the closed state by the holding D.C. current.
In this prior art, because each D.C. relay has no internal means to reduce power consumption, a supplemental relay control circuit is necessary to reduce the power consumption of the D.C relay. Therefore, when the number of D.C relays used in the control circuit increases in proportion to the function of a cooking apparatus, such as a microwave oven, there is no need for the power supplying transformer itself to be made larger, however, because each of the D.C relays requires a supplemental relay control circuit in order to reduce the power consumption, the control circuit substrate in which power supplying transformer and other electronic parts forming the control circuit are mounted must be made larger (in proportion to the function of cooking apparatus).
As a result, in this prior art, the control circuit of a cooking appratus such as a microwave oven becomes larger and more expensive as the number of the functions increases.